Anyone here a shade-tree/Hobbyist Mechanic?

[SideShow Bob]

Park on level ground or in a garage, put a drain pan under diff. Remove the fill plug. Then remove the drain plug, ( if no drain plug, loosen but don't remove diff. cover bolts and give bottom of cover a firm thump with a plastic mallet to break the seal ) let fluid drain and reinstall drain plug ( or now remove cover bolts clean surfaces, replace gasket reinstall cover and bolts and torque to specs ), fill with proper fluid or gear lube reinstall fill plug, done.
Oh yeah, think green, take old lube to recycling center.

Edit: for the transfer case you would need to drop the skid plate then remove the fill plug and drain plug unless you want fluid dripping from it for a couple of weeks.
I take my truck to have the tranny done properly, too much hassle and it can get flushed out.